In the past, the MRI phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, MRI spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, MRI has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. Thereby, energy is applied to the nuclear spins at a defined frequency, the Larmor frequency, and tissue related data is acquired at the same frequency by the dissipated energy as the nuclear spins regain their equilibrium. The use of MRI to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and radio frequency (RF) coils.
In imaging techniques using the MRI phenomenon, the subject to be imaged remains motionless. Known imaging techniques, however, span time periods of typical heart and respiratory cycles, where movement of the subject is inevitable. A known method of avoiding distortion of an MR image from biological motion, such as heart and lung movement, is to gate acquisition of MRI signals to the cyclic movement of the heart or lungs. Unfortunately, in order to gate the acquisition of MRI signals to body movement such as heart or lung motion, probes have been placed on or in close proximity to the subject. This results in probes being placed inside the bore of the main magnet, an undesirable situation since the probes may often generate distortions in the uniform magnetic field Bo and/or in the radio-frequency (RF) field, B1, with a resulting reduction in image quality. Applying probes to the patient also reduces scanner throughput, thereby increasing the cost per scan.
Physiological signals are important parameters for MRI imaging, such as for gating and triggering sequences or retrospective image correction. Respiration information measured with a pneumatic belt is dependent on the belt position and belt sensitivity. Since breathing behavior varies between patients and can change on a patient during an MRI examination, a reliable measurement can be difficult. Further, in pediatric and neonatal imaging this technique can often not be used due to the lack of special pediatric equipment (e.g. smaller belts) or the physiological differences of a newborn.
In cardiac magnetic resonance imaging (e.g. cardiac MRI), the measurement of physiological parameters is used to trigger the imaging sequence, to monitor the patient, or to collect the data for post-processing purposes. Current techniques use pneumatic belt systems to acquire respiratory motion and utilize other sensors to acquire the electrocardiogram (ECG) for measurement of the cardiac activity as to heart movement. The reliability of the ECG, however, often suffers from electromagnetic fields applied during the imaging sequence or by the electromagnetic effects on the blood. Furthermore, ECG represents the electrical activity of the heart rather than the mechanical contraction.
One contactless method utilizes a respiratory belt with wireless data transfer. Another contactless method uses the impedance change of the transmission coil caused by chest movement. This causes a shift of the resonance frequency which leads to alteration of the reflected power on the excitation frequency. This method solves the position dependence of the pneumatic belt, but is limited, however, in sampling rate and data volume, as caused by its strong dependence on the repetition time of the sequence and the number of excitation pulses. Another contactless method uses nuclear magnetic resonance (NMR) pick-up coils (PUC) to measure the impedance change independently from the imaging coil. Another contactless method is described, where a second coil inside the MRI imaging coil is used for the impedance change measurements. Thereby the second coil has a much higher frequency than the MRI imaging coil. Another contactless method is described by the usage of ultra-wideband radar (UWB), where frequencies in the GHz range are utilized.
Aside from physiological signals, bulk motion further degrades the image quality. It will therefore be useful for a system to provide overall measurement of motion that can be applied to prospective motion correction or scan interruption. The system will address improvement of signal information, including sensitivity and reliability of physiological data. As follows, the invention addresses the needs thus described.